Appended material on object doesn't render
By default, render preview uses bilt-in HDR:



If you click on use Scene World, Blender will show result similar to render one: 



So, if you have not proper environment for your model, and there is nothing is reflected in it, it looks flat.

So you should set up environment texture: 



You can use bilt-in HDRI images from Blender folder, or download any (for example from https://hdrihaven.com)